1. Field of the Invention
The present invention relates to an efficient measurement of particulate matter of a relatively small size and includes a particulate matter collecting filter for collecting suspended particulate matter, for example, in the atmosphere or various exhaust gases, a particulate matter analyzer for measuring the particulate matter on the collecting filter, and a system for automatically analyzing and collecting samples.
2. Description of Related Art
As one of the methods of measuring suspended particulate matter (hereinafter referred to as SPM) in the atmosphere, a predetermined flow of air from the atmosphere is continuously suctioned into a sampling tube as sample gas to continuously collect suspended particulate matter as dust on a tape filter in a vacuum chamber installed in the downstream side of the sampling tube. The collected dust is illuminated with β-rays from a β-ray source to detect the transmitted β-rays at that time through the collected dust by a detector to measure a mass of the collected dust by means of a β-ray absorption method.
In recent years, however, quantitative analysis has come to be required on not only a total amount of SPM but also on concentrations of individual components (elements). The dust collected on the filter is analyzed by various methods such as ion chromatography, fluorescent X-ray analysis or PIXY (charged particle induced X-ray analytical method) to thereby analyze concentrations of individual components contained in the dust quantitatively. To analyze SPM on not only a total weight (mass or concentration) but also individual components contained in SPM as described above, it is important in specifying a source of SPM to secure a meaningful sample.
In a case where SPM in the dust is quantitatively analyzed on individual components thereof, for example, by means of an ion chromatographic instrument, a fluorescent X-ray analyzer or a PIXY analyzer, it has been necessary to use a complicated procedure such as preparation of a calibration curve for a known specimen and to provide a sensitivity correction in an instrument described above before a quantitative analysis can be made on each component of the SPM.
With regard to suspended particulate matter which exits in the atmosphere, SPM having a diameter of 10 mm or less has been found to especially impair human health, while SPM having a diameter of 2.5 mm or less, which is referred to as micro particulate matter or PM2.5, is reported to have a close relationship with a higher human death rate.
As one of the techniques of measuring the mass (concentration) of the SPM in the atmosphere, for example, there is a method using a manually-operated sampler such as a low volume sampler. Specifically, the sampler is configured to collect the SPM on a filter by drawing a predetermined flow rate of the atmosphere continuously into a sampling tube for obtaining a sample gas and, then, making the sample gas pass through the filter provided in a downstream side of the sampling tube. The SPM is collected on the filter in this sampler for over a day to a few days and, after that, the mass of the particles is measured by means of a balance or the like, so that the concentration of the SPM can be determined from the mass of the collected SPM and the flow rate of the sample gas. See, for example, Japanese Unexamined Patent Publication No. 2001-343319).
With regard to a material for the filter, a fluororesin, which has a superior chemical stability and a low hygroscopicity, is mainly used. However, a filter made of a fluororesin is generally sensitive to electrification, so that such a filter has a disadvantage to absorb also unwanted matter together with the collected particulate matter due to such electrification. The absorption of the unwanted matter due to the electrification occurs during the collecting of particulate matter with the sampler, and in particular, such absorption occurs remarkably during the measurement of the mass on the collected filter which has been transferred from the sampler to a balance. This is a cause of a large measurement error in the analysis of the particulate matter.
In recent years, there has been an increasing demand for more specific composition analysis of the SPM. Therefore, a filter on which the SPM has been collected can be analyzed by means of an X-ray spectrometer for determining the composition of the SPM. See, for example, Japanese Unexamined Patent Publication No. 10-68684 (1998).
However, the above conventional method can be very time-consuming because the mass measurement of the SPM by means of a balance and the composition analysis of the SPM by means of an X-ray spectrometer are carried out separately.
Alternatively, another technique of measuring the mass of the SPM collected from the atmosphere utilizes a so-called β-ray absorption method wherein the mass of the filter on which the SPM has been collected is not measured by means of a balance, but a measuring spot formed by collecting particulate matter on a filter is irradiated with β-rays and the mass of the SPM is measured on the basis of the detection value of the transmitted rays.
However, since the filter used in the β-ray absorption method is usually made of a glass fiber containing silicon, sodium, zinc and the like which comparatively absorb X-rays, it has been difficult for the X-ray spectrometer to carry out a composition analysis of the SPM collected on the filter.